Cs + Speciation on Soil Particles by TOF-SIMS Imaging Gary S. Groenewold,* Jani C. Ingram, Travis McLing, and Anita K. Gianotto Idaho National Engineering and Environmental Laboratory, Idaho Falls, Idaho 83415-2208 Recep Avci Image and Chemical Analysis Laboratory, EPS 259, Montana State University, Bozeman, Montana 59717 Soil particles exposed to CsI solutions were analyzed by imaging time-of-flight secondary ion mass spectrometry and also by scanning electron microscopy/ energy-disper- sive X-ray spectroscopy (SEM/ EDS). The results showed that Cs + could be detected and imaged on the surface of the soil particles readily at concentrations down to 1 6 0 ppm, which corresponds to 0.04 monolayer. Imaging revealed that most of the soil surface consisted of alumi- nosilicate material. However, some of the surface was more quartzic in composition, primarily SiO 2 with little Al. It was observed that adsorbed Cs + was associated with the presence of Al on the surface of the soil particles. In contrast, in high SiO 2 areas of the soil particle where little Al was observed, little adsorbed Cs + was observed on the surface of the soil particle. Using EDS, Cs + was observed only in the most concentrated Cs + -soil system, and Cs + was clearly correlated with the presence of Al and I. These results are interpreted in terms of multiple layers of CsI forming over areas of the soil surface that contain sub- stantial Al. These observations are consistent with the hypothesis that the insertion of Al into the SiO 2 lattice results in the formation of anionic sites, which are then capable of binding cations. The characterization of natural or anthropomorphic chemicals on mineral surfaces is an important topic, because the surface chemistry exerts a large influence on chemical mobility and eventual fate. The surface chemistry can be highly variable on a microscopic scale, and this inhomogeneity can confound charac- terization efforts. Highly irregular morphological features and mineral phases that are less than 1 μm across can defeat surface characterization techniques like reflectance infrared spectroscopy, and X-ray photoelectron spectroscopy. Consequently, spectro- scopic imaging investigations of contaminants on natural mineral surfaces have been few. Cesium contamination on soil is one system for which spectroscopic information would be of high interest. 1,2 The 134 and 137 isotopes decay by γ emission and are formed in high fission yield; the 137 isotope has a moderately long half-life (30 years). The Cs isotopes comprise one of the lasting health problems from the Chernobyl accident. 3,4 Cs can be highly mobile in some environments, and geochemically it has many of the same characteristics as potassium as a consequence of similar ionic radii in solution. 2 Hence, there is motivation for understanding the interaction of Cs + with naturally occurring mineral surfaces at the molecular level. Cs sorption has been extensively investigated, primarily by using sequential extractions together with γ spectroscopy (for radioisotopes) or atomic absorption for detection. 1,3-6 This ap- proach has been applied to the study of Cs contamination on basalts and smectites, 7 clays (illite, kaolinite), 6,8 and soils. 9 Cs was shown to prefer the mineral soil horizons in high-organic soils. 10 From these studies, it has been possible to infer mecha- nistic details: Cs will tenaciously adhere to adsorption sites and can be supplanted only by K + and NH 4 + . It appears to prefer surface “defects”, which have been termed frayed edge, and wedge sites. 11-13 However, understanding of Cs-soil systems would benefit from direct spectroscopic information. The spectroscopic approach in this study utilized an imaging SIMS instrument 14 for characterization of soil particles that had been exposed to Cs + . SIMS is well suited to the analysis of Cs + because it readily forms gas-phase secondary ions. The SIMS instrument utilizes microfocused primary ion guns, achieving spatial resolutions of less than 1 μm. The ion optics transmit the secondary ions through three electrostatic sectors to a channel- plate detector, such that the spatial information is preserved. The (1) Evans, D. W.; Alberts, J. J.; Clark, R. A. Geochim. Cosmochim. Acta 1983 , 47, 1041-9. (2) Heong, C.-H.; Kim, C.-S.; Kim, S.-J., Park, S.-W. J. Environ. Sci. Health 1996 , A31, 2173-92. (3) Fawaris, B. H.; Johanson, K. J. Sci. Total Environ. 1995 , 170, 221-8. (4) Carbol, P.; Skarnemark, G.; Skalberg, M. Sci. Total Environ. 1993 , 130/ 131, 129-37. (5) Williams, T. M. Environ. Geol. 1993 , 21, 62-9. (6) Von Gunten, H. R.; Benes, P. Radiochim. Acta 1995 , 69,1-29. (7) Ames, L. L.; McGarrah, J. E.; Walker, B. A.; Salter, P. F. Chem. Geol. 1982 , 35, 205-225. (8) Desmet, G. M.; Van Loon, L. R.; Howard, B. J. Sci. Total Environ. 1991 , 100, 105-24. (9) Essington, E. H.; Fowler, E. B.; Polzer, W. L. Soil Sci. 1981 , 132, 13-8. (10) Bunzl, K.; Schimmack, W. Chemosphere 1989 , 18, 2109-20. (11) Wauters, J.; Vidal, M.; Elsen, A.; Cremers, A. Appl. Geochem. 1996 , 11, 595-9. (12) Vidal, M.; Roig, M.; Rigol, A.; Llaurado, M.; Rauret, G.; Wauters, J.; Elsen, A.; Cremers, A. Analyst 1995 , 120, 1785-91. (13) Thiry, Y.; Myttenaere, C. J. Environ. Radioact. 1993 , 18, 247-57. (14) Schueler, B.; Sander, P.; Reed, D. A. Vacuum 1990 , 41, 1661-4. Anal. Chem. 1998, 70, 534-539 534 Analytical Chemistry, Vol. 70, No. 3, February 1, 1998 S0003-2700(97)00517-9 CCC: $15.00 © 1998 American Chemical Society Published on Web 02/01/1998